Optical communication systems are a substantial constituent of communication networks. The expression "optical communication system," as used herein, relates to any system which uses optical signals to convey information across an optical waveguiding medium. Such optical systems include, but are not limited to, telecommunications systems, cable television systems, and local area networks (LANs). Optical systems are described in Gowar, Ed. Optical Communication Systems, (Prentice Hall, N.Y.) c. 1993, the disclosure of which is incorporated herein by reference.
Optical transmitters are typically available in a particular modulation format. Depending upon the transmission medium for a given optical system, it may be desirable to use a specific modulation format to enhance the transmission characteristics of the launched optical signal.
In some optical systems wavelength division multiplexing (WDM) is employed to increase the capacity of the fiber optic network. In a WDM system, plural optical channels are carried over a single waveguide, each channel being assigned a particular wavelength. To provide compatibility of the WDM format with existing networks, it is desirable to convert a signal from a received transmission wavelength to a specific channel wavelength within the WDM system. In assignee's prior patent and patent application, U.S. Pat. No. 5,504,609 and Ser. No. 08/624,269, the disclosures of which are incorporated herein by reference, a series of optical remodulators are used to take input signals from various optical transmitters and output the information onto optical channels within the channel plan of a WDM optical system. Typically, the modulation formats of the optical signals output by the optical transmitters and the remodulators is the same. However, depending upon the type of optical receivers employed and the transmission characteristics of the optical system, it may be desirable to change the modulation format of the optical transmitters to a format which is better suited to transmission in the WDM optical system.
Thus, there is a need in the art for optical transponders which can change the modulation format of incident optical signals. There is a further need in the art for improved WDM optical communication systems which can receive incoming optical transmitter signals of various modulation formats and place the information onto WDM system optical channels in a different modulation format from that of the transmitter optical signals.